This invention relates to time-lag fuses, and more particularly to time-lag fuses whose fusible element has a low fusing point overlay, e.g. an overlay of tin, which forms alloys with the base metal of which the fusible element is made, e.g. copper or silver. A low-fusing point overlay means in this context that the fusing point of the overlay metal is lower than the fusing point of the base metal, and thus reduces by well-known diffusion processes the melting point of the resulting alloy of overlay metal and base metal, i.e. the melting point of the fusible element.
The low-fusing point alloys formed by the overlay metal and the base metal of the fusible element have a derating effect on the fuse, and this derating effect is the first reason for the time-lag achieved by low fusing point overlays. Another reason lies in the fact that alloys formed by the overlay metal and the base metal such as, e.g. tin and copper, have a higher specific resistance than either of their components. Thus the aforementioned alloy-formation results in a severance of the current path through the fuse at lower temperatures and lower current intensities that would occur in the presence of a comparable mono-metallic fusible element.
Electric fuses having low melting temperature overlays that cause severing of the fusible elements have been known for many years, and subject to continuous improvements, and the present invention is concerned with one such improvement. In particular the present invention is concerned with an improvement of fuses of the kind disclosed in U.S. Pat. No. 3,291,943 to Frederick J. Kozacka, 12/13/66, TIME-LAG FUSE WITH RIBBON FUSE LINK FOLDED IN LONGITUDINAL AND IN TRANSVERSE DIRECTION. The above patent is, in turn, a continuation-in-part of the co-pending patent applications of the aforementioned Frederick J. Kozacka, Ser. No. 414,630 and Ser. No. 414,750, both filed on 11/30/64, now U.S. Pat. No. 3,260,952, 07/19/66 and U.S. Pat. No. 3,261,950, 07/19/66.
The aforementioned U.S. Pat. No. 3,291,943 describes a fuse having a casing of insulating material, filled with a pulverulent arc-quenching filler and a pair of electroconductive terminal elements closing the ends of the casing. A fusible element or ribbon fuse link of a high fusing point metal, e.g. copper, inside said casing is surrounded by said arc-quenching filler and interconnects said pair of electroconductive terminal elements. Said fuse link includes an overlay of a low fusing point metal, e.g. tin. An axially, or fusible element proper, inner portion includes flanges enclosing acute angles with the planar web portion of the fusible element proper. The aforementioned flanges define serially related points of reduced cross-sectional area. Said fusible element further includes a pair of axially outer ends or connector tabs each conductively connected to one of the terminals of the fuse. Each fusible element further includes an intermediate portion between said connector tabs and said flanges. Each of these intermediate portions has a larger cross-sectional area than any of said points of reduced cross-sectional area of said pair of flanges. Each of said pair of intermediate portions is bent at a first point to form a first loop directed toward the center of said pair of flanges. Each of said pair of intermediate portions is further bent at a second point spaced from said first point to form a second loop directed away from the center of said pair of flanges and toward one of said pair of electroconductive terminal elements.
The aforementioned intermediate portions in the above design fulfill a number of functions. They compensate for the elongations and contractions which result from the various amounts of current carried by the fusible element proper, i.e. the above flanges supporting the overlay metal. The aforementioned intermediate portions serve further as heat dams reducing the axially outflow of heat from the fusible element proper to the pair of terminal elements at the outer ends of the casing.
As the pictorial representations of the structure of U.S. Pat. No. 3,291,943 suggest, its stability is relatively low. Slight unavoidable changes in the geometry of the above referred-to intermediate flanges may change the relative orientation of the fusible element proper and that of the casing, thus affecting the desired paralellism of the longitudinal edges of the fusible element proper and that of the axis of casing or, in other words, the constancy of the relative position of the fusible element proper to that of the axis of the casing, and consequently the uniformity of the radial heat flow in a number of speciments.
Fuses according to U.S. Pat. Nos. 3,261,950 and 3,261,952 are subject to the same limitations or drawbacks.